I've been earning a bit on a regular basis since I published it 1 year and a half ago. Not enough for replacing a salary, but enough as side pocket money. I'm lucky to have a decent following, so I didn't have to market it a lot. I'm currently writing a second one which is longer and more advanced, so hopefully it will earn me a bit more.
Thanks for the reply! Do you have any practical tips you may want to share? Maybe how did you grow your audience, how you wrote your book (technologies, etc.)?
Well I grew my audience on Twitter by building open source projects. [1] I didn't really plan or aim for anything. It mostly happened.
As for technologies, I just wrote the book in markdown using Sublime Text. Then I used a markdown-to-pdf library from GitHub to generate my ebook, and voilà. Nothing fancy really.
Sorry for coming out of nowhere, but I have lots of small tutorials I could use (I teach in a local college) which I could make small ebooks out of them.. Looking for some side money..
[1] https://github.com/joaoventura/full-speed-python